Mapping Twitter Topic Networks: From Polarized Crowds to Community Clusters

Conversations on Twitter create networks with identifiable contours as people reply to and mention one another in their tweets. These conversational structures differ, depending on the subject and the people driving the conversation. Six structures are regularly observed: divided, unified, fragmented, clustered, and inward and outward hub and spoke structures. These are created as individuals choose whom to reply to or mention in their Twitter messages and the structures tell a story about the nature of the conversatio

A new cosmic microwave background constraint to primordial gravitational waves

Primordial gravitational waves (GWs) with frequencies > 10^{-15} Hz contribute to the radiation density of the Universe at the time of decoupling of the cosmic microwave background (CMB). The effects of this GW background on the CMB and matter power spectra are identical to those due to massless neutrinos, unless the initial density-perturbation amplitude for the gravitational-wave gas is non-adiabatic, as may occur if such GWs are produced during inflation or some post-inflation phase transition. In either case, current observations provide a constraint to the GW amplitude that competes with that from big-bang nucleosynthesis (BBN), although it extends to much lower frequencies (~10^{-15} Hz rather than the ~10^{-10} Hz lower limit from BBN): at 95% confidence-level, Omega_gw h^2 < 6.9 x 10^{-6} for homogeneous (i.e., non-adiabatic) initial conditions. Future CMB experiments, like Planck and CMBPol, should allow sensitivities to Omega_gw h^2 < 1.4 x 10^{-6} and Omega_gw h^2 < 5 x 10^{-7}, respectively.Comment: 5 pages, 2 figures, submitted to Phys. Rev. Let

Predictors of Increases in Alcohol Problems and Alcohol Use Disorders in Offspring in the San Diego Prospective Study.

BackgroundThe 35-year-long San Diego Prospective Study documented 2-fold increases in alcohol problems and alcohol use disorders (AUDs) in young-adult drinking offspring compared to rates in their fathers, the original probands. The current analyses use the same interviews and questionnaires at about the same age in members of the 2 generations to explore multiple potential contributors to the generational differences in adverse alcohol outcomes.MethodsUsing data from recent offspring interviews, multiple cross-generation differences in characteristics potentially related to alcohol problems were evaluated in 3 steps: first through direct comparisons across probands and offspring at about age 30; second by backward linear regression analyses of predictors of alcohol problems within each generation; and finally third through R-based bootstrapped linear regressions of differences in alcohol problems in randomly matched probands and offspring.ResultsThe analyses across the analytical approaches revealed 3 consistent predictors of higher alcohol problems in the second generation. These included the following: (i) a more robust relationship to alcohol problems for offspring with a low level of response to alcohol; (ii) higher offspring values for alcohol expectancies; and (iii) higher offspring impulsivity.ConclusionsThe availability of data across generations offered a unique perspective for studying characteristics that may have contributed to a general finding in the literature of substantial increases in alcohol problems and AUDs in recent generations. If replicated, these results could suggest approaches to be used by parents, healthcare workers, insurance companies, and industry in their efforts to mitigate the increasing rates of alcohol problems in younger generations

Investing in Biodiversity Conservation: Proceedings of a Workshop

This document presents the proceedings of a one-day Workshop on Investing in Biodiversity Conservation held at the Inter-American Development Bank in Washington, D.C., on October 28, 1996. The first part of the workshop was dedicated to the presentation of key topics on biodiversity financing by five leaders in the field. The second part of the workshop was dedicated to a discussion and exchange of ideas on the role of the IDB in investing in biodiversity conservation. Three main recommendations emerged: 1) The Bank should prepare a report on on its experience in biodiversity projects and development programs with biodiversity components; 2) A task force should be formed to work on a bio-diversity policy or strategy; 3) IDB staff should be trained to understand the biodiversity concept and its implications in project preparation and implementation.Environmental Policy, Biodiversity, Natural Resources Management

Squelched Galaxies and Dark Halos

There is accumulating evidence that the faint end of the galaxy luminosity function might be very different in different locations. The luminosity function might be rising in rich clusters and flat or declining in regions of low density. If galaxies form according to the model of hierarchical clustering then there should be many small halos compared to the number of big halos. If this theory is valid then there must be a mechanism that eliminates at least the visible component of galaxies in low density regions. A plausible mechanism is photoionization of the intergalactic medium at a time before the epoch that most dwarf galaxies form in low density regions but after the epoch of formation for similar systems that ultimately end up in rich clusters. The dynamical timescales are found to accommodate this hypothesis in a flat universe with Omega_m < 0.4. If small halos exist but simply cannot be located because they have never become the sites of significant star formation, they still might have dynamical manifestations. These manifestations are hard to identify in normal groups of galaxies because small halos do not make a significant contribution to the global mass budget. However, it could be entertained that there are clusters of halos where there are only small systems, clusters that are at the low mass end of the hierarchical tree. There may be places where only a few small galaxies managed to form, enough for us to identify and use as test probes of the potential. It turns out that such environments might be common. Four probable groups of dwarfs are identified within 5 Mpc and the assumption they are gravitationally bound suggests M/L_B ~ 300 - 1200 M_sun/L_sun, 6 +/- factor 2 times higher than typical values for groups with luminous galaxies.Comment: Accepted ApJ 569, (April 20), 2002, 12 pages, 6 figures, 1 tabl

A Miniaturized Green End-Burning Hybrid Propulsion System for CubeSats

Conventional hybrid rocket motors with thrust levels greater than 5 N rely on forced convection within the boundary layer as the primary heat-transfer mechanism for fuel vaporization. For hybrid rockets with thrust levels less than 5 N, oxidizer mass flow levels are sufficiently small that the rate of convective heat transfer is significantly reduced, and radiative heat transfer dominates the fuel vaporization mechanism. Radiative heating is a potential concern when implementing traditional hybrid rocket core burn fuel grain designs for systems with low thrust levels adequate for small satellites and CubeSats. Radiative heating causes the system to be fuel rich leading to inefficient combustion and nozzle clogging. This paper presents a novel idea of using this radiative heating phenomenon in the design of a hybrid propulsion system suitable for CubeSats and small satellites. This paper presents the test results of two fuel grain designs, the first being an end burning design and the second a “sandwich” fuel grain design. ABS, PVC, Nylon-12 and acrylic known as PMMA were used as fuel and gaseous oxygen (GOX) was used as the oxidizer during this testing campaign. These propellants provide several advantages including: benign handling properties, simplified plumbing, and greater burn efficiency over traditional monopropellant hydrazine

Proliferating cell nuclear antigen is required for loading of the SMCX/KMD5C histone demethylase onto chromatin

UNLABELLED: ABSTRACT: BACKGROUND: Histone methylation is regulated by a large number of histone methyltransferases and demethylases. The recently discovered SMCX/KMD5C demethylase has been shown to remove methyl residues from lysine 4 of histone H3 (H3K4), and constitutes an important component of the regulatory element-1-silencing transcription factor (REST) protein complex. However, little is known about the cellular mechanisms that control SMCX activity and intracellular trafficking. RESULTS: In this study, we found that small interfering RNA-mediated knockdown of proliferating cell nuclear antigen (PCNA) resulted in the reduction of the chromatin-bound SMCX fraction. We identified a PCNA-interaction protein motif (PIP box) in the SMCX protein. Using site-directed mutagenesis, we found that the amino acids of the SMCX PIP box are involved in the association of SMCX with PCNA and its interaction with chromatin. CONCLUSIONS: Our data indicate that the intracellular trafficking of SMCX is controlled by its association with PCNA

True Direction Equilibrium Flux Method Applications on Rectangular 2D Meshes

In a finite volume CFD method for unsteady flow fluxes of mass, momentum and energy are exchanged between cells over a series of small time steps. The conventional approach, which we will refer to as "direction decoupling", is to estimate fluxes across interfaces in a regular array of cells by using a one-dimensional flux expression based on the component of flow velocity normal to the interface. This means that fluxes cannot be exchanged between diagonally adjacent cells since they share no cell interface, even if the local flow conditions dictate that the fluxes should flow diagonally. The direction decoupling imposed by the numerical method requires that the fluxes reach a diagonally adjacent cell in two time-steps. Here we present a 'true direction flux method', which is an updated version of Pullin's Equilibrium Flux Method (EFM) in which fluxes are derived from kinetic theory. Previous implementations of EFM in higher dimensions have used direction decoupling as described above. In this "True Direction Equilibrium Flux Method" (TDEFM) fluxes flow not only between cells sharing an interface, but also to diagonally connecting cells, or ultimately to any cell in the grid. We compare TDEFM results to those from a direction-decoupled methods using 1D fluxes calculated with EFM and a Godunov solver. The test flow is a cylindrically symmetric implosion which we solve on a two-dimensional Cartesian grid, with cell interfaces parallel to the x and y axes. Because the flow is in theory radially symmetric, any lack of radial symmetry in the solution can be used to assess the inaccuracies in the computed results. The conventional direction decoupling methods with 1D solver flux calculations (EFM or Godunov Method) produced greater asymmetries (inaccuracies) in the solution than did the new method. TDEFM requires 1% less CPU time than the direction decoupled Riemann solver and 15% more CPU time than direction decoupled EFM